The electroluminescent device is a self-emission type device and therefore has high visibility, is excellent in display performance, can respond at a high speed, and can be reduced in thickness. Accordingly, the electroluminescent device has been attracting attention as a display device such as a flat panel display.
Of the electroluminescent device, an organic light-emitting device using an organic compound as a light-emitting body has, for example, such characteristics that the device can be driven at a lower voltage than a drive voltage of an inorganic light-emitting device, the area of the device can be easily enlarged, and a desired emission color can easily be obtained by selecting an appropriate coloring matter. Accordingly, the organic light-emitting device has been actively developed as a next-generation display.
Here, as the process of producing an organic light-emitting device using an organic light-emitting body, there are included a process involving producing a low-molecular-weight compound by a dry process such as vacuum evaporation, and a coating film formation process such as a spin coating method, a casting method, and an ink-jet method.
In a case of producing the device by the coating film formation process, the organic light-emitting device produced by the coating film formation process (hereinafter, simply referred to as “coating organic light-emitting device”) has, for example, the following merits compared to the organic light-emitting device produced by the dry process:
(1) the device can be produced at low cost;
(2) the device can be increased in area easily; and
(3) the controllability of doping in a slight amount is excellent.
FIG. 9 is a cross-sectional view illustrating a general structure of a coating organic light-emitting device. An organic light-emitting device 110 illustrated in FIG. 9 has an anode 101, a hole injection layer 102, a light-emitting layer 103, an electron injection layer 104, and a cathode 105 formed sequentially on a substrate 100.
In the organic light-emitting device 110 shown in FIG. 9, a mixture of polythiophene and polystyrene sulfonic acid (PEDOT:PSS) is generally used as a constituent material of the hole injection layer 102, and the film is formed by a spin coating method or the like. Here, the mixture PEDOT:PSS is soluble in water and insoluble in an organic solvent. Accordingly, even when the light-emitting layer 103 is formed by dissolving a constituent material of the light-emitting layer 103 in a non-polar solvent and by coating the solution on the PEDOT:PSS film, the PEDOT:PSS film is not eluted. Therefore, the PEDOT:PSS is regarded as a suitable hole injection material for production of a coating organic light-emitting device.
For the formation of the light-emitting layer 103, a polymer compound is mainly used. This is because the polymer compound has high amorphous property and therefore hardly crystallizes as compared to a low-molecular compound. Specific examples of the materials used include polymers such as polyvinyl carbazole (PVK) which is a disconjugated polymer, polyphenylene vinylene (PPV) and polyfluorene (PF) which are n-conjugated polymers, and derivatives thereof. In particular, the n-conjugated polymer is also referred to as “conductive polymer”. The polymer material which is a constituent material of the light-emitting layer 103 is formed into a solution, and then formed into a film by a spin coating method, an inkjet method or the like.
Next, the electron injection layer 104 composed of lithium fluoride or the like and a metal electrode which becomes the cathode 105 are sequentially formed on the light-emitting layer 103 by employing a vacuum evaporation method, whereby an organic light-emitting device is completed.
As described above, the coating organic light-emitting device can be produced by a simple process. Accordingly, the device is expected to find use in a wide variety of applications. However, the device involves such a problem to be solved that the device does not have a sufficient lifetime.
Various assumptions have been made about the causes for the fact that the device does not have a sufficient lifetime. One of the causes is considered to be difficulty in molecular weight control or purification of the polymer compound as a constituent material of the light-emitting layer 103.
One possible approach to solving the above problem involves use of an oligomer material the molecular weight control and purification of which can be easily performed as compared to a polymer material and which has higher amorphous property as compared to a low-molecular material. The oligomer material is excellent in purity and coating performance, and in addition, has high degree of freedom of material design, and various units such as a hole-transporting part, an electron-transporting part, and a light-emitting part can be provided at desired parts. Accordingly, the widening of the scope of material design is also included as a merit.
As examples of the application of an oligomer material to an organic light-emitting device, there are included applications disclosed in Advanced Material, S. W. Culligan et al., 2003, 15, No. 14, p 1176; J. Am. CHEM. SOC., A. L. Kanibolotsky et al., 2004, 126, p 13695; and Tetrahedron. Lett., G. L. Feng et al., 2006, 47, p 7089 and Japanese Patent Application Laid-Open No. 2003-055275.
In addition, another cause for the fact that the device does not have sufficient lifetime is considered to be that a space where electric charge is readily accumulated (space charge layer) is generated at an interface between respective layers and degrades the material.
The problem of space charge layer is solved by a well-known conventional technology, that is, by mixing a polymer or a low-molecular compound each having an electron-transporting property or a hole-transporting property in a polymer light-emitting layer to thereby improve the injectability of carriers into the light-emitting layer.
Meanwhile, the oligomer material is easily purified for achieving high purity compared to a polymer material because of having no molecular weight distribution, with the result that the lifetime of the device can be lengthened. However, it is realistic that the oligomer material has a molecular weight of about 10,000 or less from the viewpoint of synthesis of the material. Here, the oligomer material having a molecular weight of about 10,000 or less hardly causes crystallization or aggregation as compared to low-molecular materials and the stability of a film is improved. However, there is a fear of posing problems such as crystallization or aggregation when compared to polymer materials.
On the other hand, there is known a conventional technology for lengthening the lifetime of the organic light-emitting device by improving the carrier injectability as a result of mixing a plurality of materials in a light-emitting layer for the purpose of appropriately adjusting a HOMO level, a LUMO level, an electron mobility, and a hole mobility. In the case of the coating organic light-emitting device, there are known a polymer-polymer mixture type and a polymer-low molecular compound mixture type. However, in the case of the polymer-polymer mixture type, it is necessary to consider the compatibility of the polymer materials with each other, resulting in less choice of the polymers to be mixed. Accordingly, the polymer-polymer mixture type is not generic method. In addition, in the case of the polymer-low molecular compound mixture type, when the amount of the low-molecular material is increased, the low-molecular material will be crystallized or aggregated, resulting in difficulty in uniform mixing with desired amounts.